1. Technical Field
This invention generally pertains to a method of expanding a tube, and specifically to the pre-expansion of a heat exchanger tube using pressurized fluid.
2. Background Art
During the construction of tube and shell heat exchangers, it is common practice to expand a section of a tube so that it engages a bore within a tube sheet. A variety of mechanical swaging tools are available for this purpose, most involving a mandrel having rollers which may be forced radially outward as the mandrel is turned within the tube bore.
Radial force to deform the tube into contact with the bore of the tube sheet can also be provided by hydraulic fluid. Examples of this approach are disclosed in U.S. Pat. Nos. 3,979,810; 4,125,937; 4,159,564; and 4,210,991. In the U.S. Pat. No. 3,979,810 patent, a mandrel having spaced apart "O" ring seals is inserted into the tube after its installation in a tube sheet, and pressurized fluid is supplied through passages in the mandrel to expand the tube. Roll swagging of a portion of the expanded section subsequently eliminates some of the resulting tensile stress, and eliminates annular crevices at the inner edge of the tube plate bore.
The other three patents noted above disclose related tube expansion mandrels. The U.S. Pat. No. 4,125,937 patent provides for a tapered portion on the mandrel and a helical spring acting in cooperation to ease the mandrel into the tube and seat the "O" ring, once it is in position. In the U.S. Pat. No. 4,159,564 patent, an eddy current coil is used to position the mandrel before the pressurized fluid is applied. A very high pressure pulse is applied to the already pressurized annular space between a tube and a mandrel sealed with "O" rings in the U.S. Pat. No. 4,210,991 patent; this insures that the tube is fully expanded within a tube sheet bore.
None of the prior art addresses the problem of seating a tube in a tube sheet/support after a treatment to enhance heat transfer has been applied to the outer surface of the tube. It is well known that application of sintered metal or a flame sprayed porous coating to the exterior surface of a tube greatly increases its heat transfer capacity by improving nucleate boiling. Such coatings are somewhat fragile, and are likely to crack or break away if adjacent sections of the tube are expanded in a tube sheet/support after the coating is applied. Particulate matter from the coating may later contaminate the heat exchange fluid and the connected system.
The added thickness of a heat transfer enhancement coating increases the diameter of the bores in the tube supports and tube sheets through which the tube is inserted when it is installed. Consequently, the tube must be expanded to a larger diameter to seat within these bores. If roll swaged in a single operation without annealing, the tube may crack as a result of work hardening. Expansion of a tube to the relatively larger diameter required for a tube that is coated compared to one that is not greatly increases the risk that such cracking may occur.
It is therefore an object of this invention to provide a method for pre-expanding heat exchanger tubing, prior to its installation in a tube sheet/support.
A still further object of this invention is to pre-expand selected sections of tubing prior to the application of a heat transfer coating to avoid cracking or breaking away of the coating after the tube is installed in a tube sheet.
Yet a still further object of this invention is to avoid cracking of the tube when it is installed in a tube sheet/support.
These and other objects of the invention will be apparent from the description of the preferred embodiment which follows hereinbelow and from the drawings.